A linear model describing the cardiac and respiratory responses to mild work is being tested for applicability at heavier work loads. Besides the well-known augmentation of VE, out of proportion to work at heavy loads, a major nonlinearity is a progressive increase in the asymmetry of the heart rate respose. The decline in heart rate after the end of the work period becomes progressively slower, relative to the "on" response, as work level is increased. A similar prolongation of a high heart rate follows release of circulatory occlusion of the exercising limbs during light work. The time course of this phenomenon follows that of the augmented ventilation after release of the occluding cuffs. The increase in ventilation is associated with and follows (after a brief, approximately 5 sec, delay) an increase in end-tidal CO2. Both circumstances of elevated heart rate (post-occlusion and following heavy exercise) may be due to accumulation and subsequent gradual wash-out of metabolites which cause excitation of chemoreceptors in the muscle. The rapid ventilatory response to a brief pulse of exercise is increased during occlusion of the exercising limbs. Experiments are now being conducted to determine whether this potentiation of VE is caused by increased cortical drive or by metabolite stimulation of muscle receptors.